The present invention relates to an image recording and reproducing apparatus for recording/reproducing image data and, in particular, image data used for medical diagnosis.
In a CT (computed tomography apparatus) known as a radiation tomographic imaging apparatus, tomographic image data of an object under examination is converted into display data and is displayed on a television (TV) monitor. In an apparatus of this type, a means is required for recording digital image data on a recording medium such as a magnetic disk or optical disk, and for reproducing and displaying the recorded image data.
This type of image data is expressed using a finite number of grid-like spaces, i.e., an image matrix (to be referred to as matrix hereinafter). Image data consists of gradation data for each pixel of the matrix. In general, the resolution of an image depends on the number of pixels in the image, i.e., the matrix size. For this reason, in order to record digital image data having a high resolution and a large number of gray levels using a CT apparatus or the like, a recording medium having a large capacity must be provided. For example, when the matrix size is 320.times.320, the number of pixels of the image is 102,400.
Such image data is recorded or reproduced in units of images, i.e., using all the pixels in the matrix. This requires a recording medium having a large capacity, which corresponds to the matrix size, as described above.
Various proposals have been conventionally made to reduce the recording capacity required for image recording and to allow recording of a large amount of image data on a medium having the smallest possible recording capacity. As disclosed in Japanese Patent Disclosure (Kokai) No. 58- 159731, a portion of obtained image data which is not required for diagnosis, e.g., a portion corresponding to air in a specific object portion under examination, is removed and data of only effective region of the object is recorded, thereby allowing reduction in the required recording capacity.
Recently, in order to obtain an image suitable for medical diagnosis, the slice width (thickness) in a CT apparatus has become thinner. With this trend, the number of images handled by the apparatus has increased. In actual diagnosis, a predetermined number of tomographic images are obtained at predetermined intervals. This predetermined number has also increased. In order to cope with such an increase in the amount of image data to be processed, removal of data portions corresponding to air in the object is not sufficient. Another measure must be taken to allow recording of a larger amount of image data on a medium having a small recording capacity.